This invention relates to the preparation of metal sulfides, and, more particularly, to the preparation of high purity sulfides and doped sulfides suitable for luminescense applications.
Luminescence occurs when certain materials are exposed to light or other energy, termed photoluminescence, or excited with an applied electric field, termed electroluminescence. Luminescence is widely used in a variety of commercial and scientific applications. For example, conventional television screens and cathode ray tubes are coated with photoluminescent phosphors which produce light when irradiated with electrons directed against the back side of the screen by the electron gun of the picture tube. Luminescent materials and devices also have application in detectors, instruments, and displays, among other things.
Not all materials can produce luminescence, and the identification and development of improved luminescent materials is ongoing. The most widely used luminescent materials to date have been rare earth oxysulfides doped with other rare earth elements. As an example, Y.sub.2 O.sub.2 S doped with less than one percent of europium is widely used as the red light emitting phosphor in television tubes. As in most luminescent materials, there is a host material in which is dopant species resides. The dopant actually undergoes the electronic transitions that produce light, but the character of the host is important in creating an environment for the luminescing species so that luminescence can occur and not be quenched by impurities.
Other classes of materials are under consideration for their luminescence, such as metal sulfides doped with metallic or rare earth cations. These materials have the advantage that they can be excited or driven to higher light outputs than doped rare earth oxysulfides, with high light output efficiency and without a breakdown of the material. Additionally, metal sulfides are less expensive than rare-earth-based hosts.
The key to the effectiveness of any luminescent material or device is its ability to produce a high light output under a particular level of excitation, its efficiency, and to be driven to high light output without degradation of the material. A principal obstacle to reaching these goals is the presence of certain types of impurities in the host lattice. Specifically, hydrogen-containing impurities in the lattice of doped metal sulfides have inhibited their widespread use in many applications, because the impurities interfere with the production of light by the dopant and reduce the threshold of radiation damage of the material. Consequently, this class of luminescent materials has not been widely exploited in devices for which it is otherwise ideal, in spite of its potential advantages.
There are a number of techniques now used for producing metal sulfides and doped metal sulfides commercially. In one method, sulfur is boiled in a thick aqueous suspension of calcium hydroxide. A metal sulfide is precipitated, but the ratio of metal to sulfide ions varies widely from batch to batch, and is not readily controlled. Moreover, hydroxide ions are inherently present to contaminate the solid. In another method, solid state conversion of metal sulfates is accomplished with reduction by carbon. This approach is used to prepare metal sulfides for industrial applications where high purity is not required, such as insecticides and depilatories. The resulting impurity content of the resulting metal sulfide is simply too high for its use in luminescence.
In another process, the metal oxide is fused with sodium carbonate in the presence of an excess of sulfur. The metal sulfide is precipitated, with evolution of sulfur dioxide. The free energy of the reaction is positive, but the reaction can be coaxed along by continuously removing the products. This process does not avoid the hydrogen-containing impurities, as oxide in the reaction and carbonate present in the flux can react with any available moisture to produce hydroxide.
The potentially most satisfactory of the prior methods for producing metal sulfides is the conversion of metal chlorides or oxides with hydrogen sulfide or ammonium sulfide. Different variations of this approach are used, some in aqueous solution and others at elevated temperature with the chloride or oxide in the solid state. When accomplished in aqueous solution, there is an inevitable contamination of the metal sulfide with hydrogen in the form of incorporated hydroxide and bisulfide ions. Each solid state reaction has a positive free energy change, and elevated temperature or removal of products are used to encourage the reaction. However, the product remains contaminated with a level of the chloride or oxide, due to the incomplete reaction. In turn, due to the ubiquitous presence of water vapor these impurities form the hydroxide ions that remain to contaminate the metal sulfide.
To summarize, there has been publicly proposed no alternate approach for producing metal sulfides and doped metal sulfides of suitably high purity for use in luminescence applications, although several supplier companies apparently utilize proprietary processing. As a result, this potentially attractive class of luminescent materials has not been exploited. There therefore exists a need for a process for economically and commercially preparing metal sulfides and doped metal sulfides of sufficiently high purity for use in luminescence. Any such process should specifically result in low oxygen and hydroxide contamination of the sulfide. The present invention fulfills this need, and further provides related advantages.